refactor(daemon): share the output ring-window helper

The embedded daemon and the xmrig manager each kept their own copy of
the console output ring: append new bytes, trim the front to a cap at a
newline boundary, and hand a reader everything since its offset. The two
copies had drifted. The daemon tracked an absolute byte total so a
reader's offset stayed valid across a front-trim, while the miner keyed
its reader off the live buffer size, so a trim could silently rewind the
console feed.

- The append/trim and the since-offset logic now live in one small
  header, outputWindowSince and appendBounded, included by both. The
  trim keeps the newline-boundary cut and the guard that stops a lone
  trailing newline from erasing the whole buffer.
- The xmrig manager now carries the same absolute byte total, so its
  reader resolves against the window the same way and survives a trim
  (and a process restart that clears the buffer but keeps the total).

Tests cover feeding the ring well past the trim point while a reader
polls each chunk (the feed is continuous and never dies at the trim),
the lone-trailing-newline guard, and a cleared buffer with a retained
total.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/daemon/xmrig_manager.h

@@ -11,6 +11,8 @@
 #include <vector>
 #include <cstdint>
 
+#include "output_window.h"
+
 #ifdef _WIN32
 #include <winsock2.h>
 #include <windows.h>
@@ -106,13 +108,7 @@ public:
      */
     std::string getOutputSince(size_t& offset) const {
         std::lock_guard<std::mutex> lk(output_mutex_);
-        if (offset >= process_output_.size()) {
-            offset = process_output_.size();
-            return {};
-        }
-        std::string result = process_output_.substr(offset);
-        offset = process_output_.size();
-        return result;
+        return outputWindowSince(process_output_, output_total_, offset);
     }
 
     /**
@@ -169,6 +165,7 @@ private:
 
     mutable std::mutex output_mutex_;
     std::string process_output_;
+    std::size_t output_total_ = 0;     // bytes ever appended (absolute end; guarded by output_mutex_, kept across restarts)
 
     // xmrig HTTP API credentials (random per session)
     int         api_port_  = 0;